It is already known to utilize optical sensing heads on coordinate measuring apparatus in lieu of the mechanical sensing head which is conventionally used and with which the workpiece to be measured must be contacted. It is also known to use the optical sensing heads in addition to this mechanical sensing head. The optical sensing heads permit a contactless detection of the geometric elements to be measured. For this purpose, optical sensing heads which make point measurements are used which, as a rule, function according to the so-called triangulation principle. Also, so-called video sensors can be used from which a more or less complete image of the geometric element to be measured is recorded and later evaluated with known means of image analysis.
The illumination of the object details to be measured presents special problems in video sensing systems of the last-mentioned type. This condition is present because the angle at which the illuminating device radiates must be adapted to the focal length of the objective used to obtain a reflection-free illumination rich in contrast especially for smooth or metallic workpiece surfaces. This is often only obtainable with great effort. Furthermore, heat losses arise with most of the light sources used which can lead to an undesired deformation of the machine coordinate system.
It has already been suggested to utilize fiber optic ring illumination in combination with video sensors in coordinate measuring apparatus in order to avoid the above-mentioned problem. For example, U.S. Pat. No. 4,706,168 discloses a coordinate measuring apparatus having such a video sensor. However, here the supply for the ring illumination is provided via flexible light conductors from a stationary light source. A complicated adjusting mechanism is necessary for adapting the illumination angle to the focal length of the objective used. It is not possible to adjust the video sensor in different viewing directions.
It has also already been suggested to attach optical sensing heads such as triangulation sensors or video sensors via a so-called rotational-pivot joint on the measuring arm of a coordinate measuring apparatus in order to realize different viewing directions. Such a sensing system is described, for example, in U.S. Pat. No. 4,688,184. However, this known sensing system is very expensive because a rotational-pivot joint which can position a video sensor with its relatively high mass with great precision requires a very complex structure and a complex manufacturing procedure.
International patent application No. WO 87/01798 discloses a sensor change device which makes it possible to exchange mechanical sensing heads for optical sensing heads. However, this patent application provides no suggestion with respect to a configuration of an illuminating device for optical sensors.